Question about DOF in MF.

Is there a big difference in DOF as you change film or sensor sizes? I noticed a little bit when comparing 35mm to DX size sensor in my DX DSLR. But will there be a huge difference if I go to, say 6X4.5 or 6X7? Many times it's frustrating with the DSLR-you need to be 24mm or shorter to get a wide angle perspective. And sometimes it's too much DOF, even wide open.

You get a considerably large difference in DoF with medium format. I was very surprised the first time I developed a 6x6 negative and saw how little DoF it had compared to the 35mm that I was shooting before.

DOF depends on the focal length and aperture. With MF you will use longer lenses to get the same angle of view which means less DOF.
A lens with a certain focal length will give the same DOF whatever camera it is mounted on but as you move up i format it will give a wider angle of view (as far as it will cover).

There is a considerable difference in DOF if you switch to larger formats. For example, a 50mm lens at f5.6 on 35mm has a hyperfocal distance of 14.8 meters. On 6x7, the same focal length and aperture gives a hyperfocal distance of only 7.42m.

Why use a "tool" when the computations are so simple (and then you understand what you do) ?

The following simple formula :

h = f² / Dc

yields the hyperfocal distance h given the focal length f, the f/stop D and the admitted "circle of confusion" c (your choice ! depending for instance on how much you want to enlarge your pictures)... The hyperfocal distance being the DOF to or from infinity : to get an idea of DOF at nearer distances, you simply have to translate this on the focusing ring of the lens.

nor with this :

There is a considerable difference in DOF if you switch to larger formats. For example, a 50mm lens at f5.6 on 35mm has a hyperfocal distance of 14.8 meters. On 6x7, the same focal length and aperture gives a hyperfocal distance of only 7.42m.

Because this would mean that you have more DOF (a nearer hyperfocal distance)with larger format ? Actually on a larger neg you may admit larger circles of confusion (or maybe not, depending on your choice). But let's suppose you do : 72mm is two times 36mm, so you may take 2c instead of c and thus get h/2 instead of h with the same D and the same f. OK !

But actually to compare pictures with the same perspective, i.e. with the same angle of view : with the large format camera, you would use a 100mm lens instead of the 50mm lens. So instead of f you use 2f , instead of f² you get 4f² and thus, even with 2c instead of c, you finally get 2h instead of h (or h/2). What everybody (?) agrees with : large format yields less DOF.

Paul

Last edited by polka; 09-04-2011 at 07:32 AM. Click to view previous post history.

The rule of thumb for distant objects is that DOF stay the same across formats as long as the linear aperture is constant. As you focus closer, this relationship breaks down, so that at macro distances, large format has much less DOF at the same linear aperture.

Why use a "tool" when the computations are so simple (and then you understand what you do) ?

The following simple formula :

h = f² / Dc

yields the hyperfocal distance h given the focal length f, the f/stop D and the admitted "circle of confusion" c (your choice ! depending for instance on how much you want to enlarge your pictures)... The hyperfocal distance being the DOF to or from infinity : to get an idea of DOF at nearer distances, you simply have to translate this on the focusing ring of the lens.

nor with this :

Because this would mean that you have more DOF (a nearer hyperfocal distance)with larger format ? Actually on a larger neg you may admit larger circles of confusion (or maybe not, depending on your choice). But let's suppose you do : 72mm is two times 36mm, so you may take 2c instead of c and thus get h/2 instead of h with the same D and the same f. OK !

But actually to compare pictures with the same perspective, i.e. with the same angle of view : with the large format camera, you would use a 100mm lens instead of the 50mm lens. So instead of f you use 2f , instead of f² you get 4f² and thus, even with 2c instead of c, you finally get 2h instead of h (or h/2). What everybody (?) agrees with : large format yields less DOF.

Paul

I still think that a tool is easier to make calculations, but suit yourself. You're completely right about the second quote, I must have had a mind fart. Sorry about that

Polka, I'd rather not carry around a calculator or portable computer to do the calc for each shot. By the time it is done, the critical moment may have passed. I'd rather use the DoF scale on the lens. Happily, I do not use zoom lenses except on my DSLR or any lens that does not have a DoF scale on it. For the DSLR, I seldom use it or just use it for the same type of photos as I would a disposable. We should pressure companies to put the scales back on the lenses. In the cave days of film zooms sported scales so it is not impossible.

Fortunately, I do not need any calculator or computer to do (within about 10% accuracy) this simple formula that I showed, and I bring with me my head all the time - even more often than my camera. But you are right, the DOF scales on the lenses are the quickest way. However, depending on makers or brands (or even lenses form the same origin or use) they give very dissimilar results, thus it may be wise to use the formula only once for each of your lenses, to check what circle of confusion it surmises.

For instance, looking at the lenses I have for my ARAX88 (Kiev), I find that for the 45mm/3,5 (Mir), the 150mm/2,8 (Kaleinar) and the 250mm/3,5 (Jupiter), the Arsenal poeple surmise a circle of confusion c = 0.15mm, whereas for both 80mm/2,8 (Arsat and CZJ Biometar) c = 0.06mm - which seems more suitable for MF.

And even that value may not satisfy your needs : if you think your enlargments require c = 0.045, you will have to close the ARSAT or Biometar one stop more than shown by their DOF scales, and the other Kiev lenses 3 stops more !